1. Field of the Invention
The present invention relates to a high pressure discharge lamp. It relates in particular to a high pressure discharge lamp that is characterized by electrodes inside a discharge tube.
2. Description of the Prior Art
In general, a conventional electrode inside a discharge tube of a high pressure discharge lamp includes an electrode pin made of tungsten and an electrode coil made of tungsten that surrounds the electrode pin.
The known electrode coil is formed by which a wire having a smaller wire diameter than that of the electrode pin is coiled to make one or two layers around the electrode pin. Further known structures are that with a tip of the electrode pin located in the electrode coil and that with the tip of the electrode pin being in the same plane as or protruding beyond an end of the electrode coil in the central side of the discharge tube.
The purpose of providing the electrode coil to the electrode pin is to lower the temperature of the tip of the electrode pin during the discharge so as to achieve an optimal temperature, thereby reducing the evaporation of tungsten on the tip of the electrode pin and suppressing the blackening of the discharge tube. It is another important purpose to form the electrode coil with a thin wire, thereby increasing the electrolytic strength and improving the startup performance of the lamp.
In order to optimize the tip temperature of the electrode pin to improve its lifetime characteristics, studies have been conducted on the wire diameters of the electrode pin and the electrode coil, and the coiling number and shape of the electrode coil.
In such a conventional electrode of the high pressure discharge lamp, the electrode pin and the electrode coil are fixed to each other by welding or a mechanical insertion (that is, a method for obtaining the fitting strength by making the inner diameter of the electrode coil slightly smaller than the outer diameter of the electrode pin and mechanically pushing the electrode pin into the electrode coil). However, since the electrode coil is formed with a wire, the contact area of the electrode pin and the electrode coil is very small, and thus is less effective in lowering the tip temperature of the electrode pin.
Moreover, because the contact area varies considerably depending on electrodes, the tip temperature of the electrode pin varies correspondingly. This generates a variation of the evaporation degree of tungsten on the tip of the electrode. When an evaporation amount of tungsten is great, leading to a large deterioration of the electrode tip, the evaporated tungsten adheres to the inner surface of a discharge tube so that the discharge tube blackens, thus considerably reducing a light flux of the lamp. Furthermore, higher temperature of the discharge tube and higher lamp voltage are caused so as to accelerate the dying out of the lamp. Moreover, when the electrode coil is formed by dense coiling and gaps occur between coil pitches, then the thermal conduction is greatly lowered, making the above problems still more serious.
It is an object of the present invention to solve the problems mentioned above and to provide a high pressure discharge lamp that has excellent lifetime characteristics and can considerably reduce the variation of the lifetime characteristics between lamps.
In order to achieve the above-mentioned object, the present invention has the following configuration.
A high pressure discharge lamp according to the present invention includes a discharge tube, a pair of electrodes comprising an electrode pin and a metal pipe surrounding the electrode pin arranged inside the discharge tube, and mercury sealed in the discharge tube.
With this configuration, a contact area of the electrode pin and the inner surface of the metal pipe is maintained sufficiently, thereby sufficiently lowering a tip temperature of the electrode pin. Also, since the contact areas in the electrodes are uniform, a variation of the tip temperatures of the electrode pins can be reduced, thereby reducing the variation of the electrode temperatures of the lamps. As a result, it is possible to provide the high pressure discharge lamp that has excellent lifetime characteristics and considerably can reduce the variation of the lifetime characteristics between lamps.